The present disclosure relates to nanostructured ferritic alloys (NFAs), and more particularly, methods for processing the same so that articles comprising the NFAs and so processed are suitable for use in challenging environments.
Gas turbines operate in extreme environments, exposing the turbine components, especially those in the turbine hot section, to high operating temperatures and stresses. In order for the turbine components to endure these conditions, they are necessarily manufactured from a material capable of withstanding these severe conditions. Superalloys have been used in these demanding applications because they maintain their strength at up to 90% of their melting temperature and have excellent environmental resistance. Nickel-based superalloys, in particular, have been used extensively throughout gas turbine engines, e.g., in turbine blade, nozzle, wheel, spacer, disk, spool, blisk, and shroud applications. However, designs for improved gas turbine performance require alloys with even higher temperature capability.
Nickel base superalloys used in heavy-duty turbine components require specific processing steps in order to achieve the desired mechanical properties. This process, referred to as a cast and wrought (C&W) approach begins with three melting operations: vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). The initial VIM operation mixes the elements together forming the alloy of interest; however, significant impurities and macro scale chemical segregation are present. The subsequent ESR and VAR steps are required to produce a chemically pure, homogeneous ingot. The grains of the resulting VAR ingot are too coarse to yield the necessary mechanical properties. As a result, the ingot is broken down via a double upset and draw operation resulting in the recrystallization and refinement of the nickel base superalloy structure. Finally the billet is forged and machined into its final desired shape.
Nanostructured ferritic alloys (NFAs) are an emerging class of alloys that exhibit exceptional high temperature properties, thought to be derived from nanometer-sized oxide clusters that precipitate during hot consolidation following a mechanical alloying step. These oxide clusters are present to high temperatures, providing a strong, stable, microstructure during service. Unlike many nickel base superalloys that require the C&W process to be followed to obtain necessary properties, NFAs are manufactured via a different processing route. Like the C&W process, the alloy chemistry is created via a VIM operation. However, following the initial melting, the NFA is atomized and collected as solid powder particles. These powder particles are then combined with an oxide additive and milled in the presence of steel shot until the oxide addition dissolves in the metal matrix. The ESR and VAR steps are not required.
In order for any material to be optimally useful in, e.g., large hot section components of heavy duty turbo machinery, it may also desirably exhibit both an acceptable continuous cycle fatigue crack growth rate as well as an acceptable hold time fatigue crack growth rate. Such a material may also desirably be utilized for smaller turbomachinery components, e.g., discs for use in jet engines, which likely have a different set of desired or required properties. Any such alloy will also desirably be capable of being manufactured into the desired article utilizing a less energy intensive and/or time consuming process, than the conventional cast and wrought process.